An electronic whiteboard system is a processor-based computing system used to input and output information associated with a software application running on the system. Typically, in accordance with such a system, one or more users “write” on the whiteboard using an electronic writing instrument or marker, such as a lightpen. The lightpen permits the user to write with “electronic ink.” Electronic ink is the term given to writing that is electronically captured from and/or electronically projected on the whiteboard without using physical ink. A user's writing, as well as any other desired information, is displayed on the whiteboard which is viewable by the one or more users. The data entered on the whiteboard may then be stored for subsequent use by the application being run on the system. Examples of such whiteboard systems are: Ideaboard by 3M Inc. (http://www.3m.com/vsd/ams/11_whtbd.html); e-Beam by Electronics for Imaging, Inc. (http://www.e-beam.com/index_flash.html); SoftBoard by Microfield Graphics, Inc. (http://www.softboard.com/); SMART Board (htttp://www.smartboard.co.uk/product/index.html); Mimio by Virtual Ink Inc. (http://www.virtual-ink.com/ns.shtml); and Liveboard, The Office of the Future: Xerox PARC, Wendy Taylor, PC Computing, pp. 192, January 1995.
Electronic whiteboard systems may be collaborative. A collaborative whiteboard system is a distributed computing system which includes two or more individual electronic whiteboard systems, as mentioned above, in communication with each other while running a collaborative application. While the individual systems, and thus their respective users, are remote from one another, a user at a first location is able to view information written by a user at a second location. This way, the remote users may interact as if they are in the same location. Examples of such whiteboard systems are: Netmeeting by Microsoft, Inc.; and Sametime by Lotus, Inc. (an IBM company).
Whether a single stand-alone system or a collaborative system, a typical electronic front-projection whiteboard system, as illustrated in FIG. 1A, is comprised of a whiteboard screen 2, a writing stylus or lightpen 4, a fixed-position projector 6, a fixed-position camera 8 and a processing system 10. In such a system, the function of projecting images representing a user's writing on the whiteboard screen 2, in accordance with the lightpen 4, is performed by the fixed-position projector 6. As shown in FIG. 1A, the projector 6 has its own imaging optics 7 associated therewith. The fixed-position camera 8, aimed at the writing substrate 2 and the lightpen 4, captures an image of the whiteboard and the light emitted by a lamp associated with the lightpen. Like the projector 6, the camera 8 has its own imaging optics 9 associated therewith. Suitable optical and electronic filtering assure that only the lightpen is sensed among possible background clutter and distractions such as other bright objects. As is known, the presence and location of the lamp of the lightpen in the field of view of the camera may be estimated by various signal processing techniques. Several technologies exist that address the problem of capturing the location of a stylus on a whiteboard. For example, one such technique is used in Xerox's LiveBoard as mentioned in S. Elrod et al., “Liveboard: A Large Interactive Display Supporting Group Meetings, Presentations and Remote Collaboration,” CHI '92 May 3-7, 1992, pp. 599-607.
The images projected by the camera on the screen, representing the user's writing strokes, are derived from a display screen buffer. The contents of the display screen buffer depend on optical screen marking events such as those generated by the lightpen. The visual effect that the user's strokes are physically being written on the whiteboard is achieved by the camera projecting the image of the optical marker or lightpen path onto the board.
As is known, the processing system 10 includes processor and memory resources for coordinating the functions performed by the whiteboard screen 2, the lightpen 4, the projector 6 and the camera 8. Accordingly, the system must accurately sense the location of the lightpen on the board and then project its writing actions onto the board. One method for accomplishing these tasks is as follows. The camera and its imaging optics are aimed at the board in order to capture the optical emission from the lightpen. The captured position of the light must then be transformed such that the projected writing trace generated by the projector appears at the tip of the lightpen as it writes. The transformation used to achieve this goal depends on many factors such as the settings and location of the imaging optics of the projector, and the settings and location of the imaging optics of the camera. However, determining the transformation can be a problem.
Such problem associated with determining the transformation can be generically described as follows. A processor produces an image that is being projected onto a physical surface. Find the spatial relationship of a visual marker, which is not necessarily controlled by the processor, relative to the projected image, for fixed optics and settings of the projector. Consider the case in which the marker is observed by a camera. Once this relationship is found, the processor that drives the projector uses the information about the marker to produce visual effects such as mimicking the action of the lightpen on the board.
In accordance with existing whiteboard projector/camera arrangements, the problem is typically solved by employing a calibration procedure, wherein the camera and its optics are calibrated with the projector and its optics so as to determine the proper transformation. Unfortunately, such a calibration procedure is typically disruptive, time consuming and usually ignores lens radial distortion and other hard-to-correct optical lens aberrations.
The same problems also exists in rear-projection whiteboard system. A conventional rear-projection whiteboard system is illustrated in FIG. 1B. The system is comprised of a whiteboard screen 2′, an enclosure 3, a writing stylus or lightpen 4′, a reflecting mirror 5, a fixed-position projector 6′, a fixed-position camera 8′ and a processing system 10′. The components and their functions in the rear-projection system in FIG. 1B are essentially the same as those in the front-projection system in FIG. 1A, as described above, with the following exceptions. In the front-projection system, the user is on the same side of the whiteboard screen as the projector, while in the rear-projection system, the user and the projector are on opposite sides of the screen. Also, the screen 2′ in the rear-projection system is typically translucent so that the lightpen 4′ can be tracked by the camera 8′, via the reflecting mirror 5, and so that the user on one side of the screen can view the images projected on the other side of the screen by the projector 6′, via the mirror 5. Like the conventional front-projection system, the projector and camera of the rear-projection system each have their own separate imaging optics 7′ and 9′, respectively. Thus, a similar calibration procedure must be performed to determine the appropriate transformation relationship which, as mentioned above, presents many operational drawbacks.
Thus, it would be highly desirable to solve the problem associated with determining the above-described transformation relationship associated with the projector and camera of an electronic whiteboard system such that the disadvantages associated with the use of a calibration procedure could be avoided.